In the thwarted terrorist attack in Germany, airports and aeroplanes once again figured, following a similarly abortive attack at Glasgow airport. The police in Britain say they have prevented several other plots to hijack and bring down aircraft. Most of the debate about the German episode has centred upon the fact that, as in the UK, "home-grown" suspects were involved. But it is worth also asking a different, and more generic, question: why do terrorists so often target airports and air travellers?

There are many other targets that could cause equivalent or greater damage in terms of loss of life. Airports and planes today are perhaps the most heavily protected environments of any. Sports stadiums, crowded shopping centres, pop festivals, cruise ships and traffic tunnels are far more vulnerable. The two most destructive acts of jihadist terrorism that have occurred in Europe so far, in Spain and the UK, have actually involved trains.

There are some obvious strategic reasons why a terrorist group might still decide to target aircraft and their passengers. Even a realistic terrorist threat is disruptive for the airline industry and at a minimum causes inconvenience for large numbers of people. Individuals who are anxious about flying might cancel their journeys indefinitely. An attack where several planes were brought down simultaneously - as was planned by the terrorist network in the UK last year - would have damaging economic effects stretching well beyond the immediate industry itself.

Yet my feeling is that there is a lot more involved than these factors alone. Most terrorist violence is symbolic. It aims to change things through affecting public opinion, frightening people and demonstrating their vulnerability. The traditional terrorism of the IRA and Basque nationalists was local in nature, and concerned with specific ends, revolving around nationhood. The symbolic element in jihadist violence is more predominant than in old-style terrorism, because its goals are nothing like as limited and clear-cut. And it is in these symbols that we find a large part of the attraction of targeting airports and planes.

One factor is no doubt the mystique that has built up around the events of 9/11. Because of the large-scale loss of life, the symbolic significance of 9/11 has not always been appreciated. The attacks were aimed at the three main symbols of American global power: Wall Street, the Pentagon and either the White House or the Capitol building.

The events of 9/11 have spanned a variety of movies and television exposés, some concentrating on the victims, some on those who organised the plot. When the attacks happened, many could not quite believe in their reality, since there have been so many disaster movies featuring similar scenarios. It was a global media event: hundreds of millions of people watched the plane crash into the second tower in real time. There can be very few in the world who did not see pictures of the destruction of the twin towers subsequently. No one is immune from these images or their dramatic appeal, although of course people can draw quite contrary and complex implications from them.

Those ready to die for a cause are not immune to the retrospective prestige the act brings them - reward in heaven may not be the only motive. A few years ago, a study was done of people who jump off of the Golden Gate Bridge in northern California. Hundreds of individuals have killed themselves in this way since the bridge was built in the 1930's. You have to be serious about killing yourself to make the jump, since very few who do so survive.

A tiny proportion does come through against the odds, however, and some of these have later been interviewed. One of the findings - confirmed in other studies - is that it matters a great deal to most people, not just that they kill themselves, but where and how they do it. The Golden Gate has an appeal and a prominence that the other main bridge across the San Francisco Bay, the Bay Bridge, does not have. Virtually no one jumps off the Bay Bridge, which is mundane and inelegant.

Targeting planes similarly has a certain glamour and perhaps all the security precautions add a perverse challenge? Killing people on trains, in supermarkets, road tunnels or crowded shopping centres is perhaps like jumping off the Bay Bridge: it just doesn't have the same appeal to jihadist action heroes. I believe that jihadist fundamentalism has a tortured and ambivalent attitude towards modernity. The object of such fundamentalism is to attack the decadence of the west and its influence over the rest of the world. Yet it is also seduced by the technologies and lifestyles to which it takes such violent exception. Modern communications, including not only jet travel, but the internet, mobile phones, television and DVDs are the stock in trade of the jihadist revolutionary.

What follows in practical terms? One point is that, in understanding the psychology of new-style terrorism, we shouldn't concentrate solely on the religious or political motives that prompt such behaviour. Other important elements may be involved, especially for young men determined to express bravery and derring-do, even in situations where, if they are successful, they will die.

I don't think all terrorist missions in the west will focus on aeroplanes. After all, in the Middle East, suicide bombers attack many quite mundane targets. But we can get some understanding of why suicide attacks against airplanes are unlikely to stop. It certainly doesn't mean that security shouldn't be as tight as it could possibly be, without undermining the convenience of air travel altogether. It is likely, however, that terrorists will target the high-profile carriers rather than the more obscure ones. And - just like in the case of persuading people to stop smoking - in some cases the very awareness of high levels of risk may act as a perverse incentive.